The invention relates to a method to optically measure the filling level of a liquid in a cavity having a sub-mm wide opening with the aid of a measuring sensor based on chromatic coding, the measuring sensor delivering a distance value and an intensity value as its output signal. The method is characterized by the steps:

Recording a distance profile and an intensity profile of the surface of the liquid by moving the measuring sensor along the opening of the cavity holding the liquid, the movement being made essentially parallel to the surface of the fluid along a measurement segment x, and

determining the filling level by means of a combined analysis of the recorded distance profile and intensity profile.